This invention relates to an exhaust system with HC adsorber and parallel exhaust gas catalytic converter and a vehicle that comprises such a system.
The use of catalytic converters in exhaust systems of internal combustion engines has become a matter of course today. For example, oxidation catalytic converters that convert unburned hydrocarbons (HC) and carbon monoxide (CO) are used for diesel engines in particular, and reduction catalytic converters that convert nitrogen oxides (NOx) in both diesel and spark-ignition engines. In addition, three-way catalytic converters are known that combine the functions of oxidation and reduction catalytic converters and thus catalytically convert all three components; these are primarily used with spark-ignition engines. All catalytic converters generally require a specific minimum temperature called light-off temperature at which they convert 50% of the limited exhaust gas components. This temperature is typically not yet reached directly after a cold start of the engine, so that emissions called startup emissions leave the exhaust system unconverted unless other steps are taken.
Current and, of course, future exhaust gas legislation requires that the startup emissions measured in standardized test cycles are included in the determination of the overall emissions of a vehicle. The desire for further emission reduction and the increasingly dropping exhaust gas limits also necessitate that startup emissions are reduced as well and thus that the catalyst system reaches its operating temperature faster.
A common measure to reduce startup emissions is provide relatively small-volume preconverters, also called primary catalytic converters, at the hot end of the exhaust system. The preconverters reach their light-off temperature relatively fast due to their small volume and their hot end location, and they take over the conversion of most of the emissions until a downstream main catalytic converter has reached its operating temperature as well.
DE 100 21 421 A1 discloses an exhaust system in which an exhaust turbine of an exhaust turbocharger is provided in the main duct of the exhaust duct and can be bypassed via a parallel bypass duct. A three-way catalytic converter or a preconverter designed as a HC adsorber is provided in the bypass duct. A controllable flap can either seal the bypass duct or the main duct shut; intermediate positions of the flap may be provided. After a cold start, the entire exhaust gas flow is conducted through the bypass duct via the preconverter. As soon as a downstream primary catalytic converter has reached its light-off temperature, the flap is switched over and the exhaust gas flow is conducted via the exhaust turbine in the main duct.
US 2002/0132726 A1 describes an exhaust system that comprises a main catalytic converter and two parallel exhaust passages upstream of it, which can be selectively closed or opened using a switchover valve. The two parallel exhaust passages are arranged concentrically, wherein the main passage is at the center and the secondary passage in which an annular HC adsorber is located concentrically encompasses the main passage. A return passage that feeds unburned hydrocarbons desorbed from the adsorber back into the internal combustion engine branches off the secondary passage upstream of the HC adsorber. After a cold start, the interior main passage is closed and the exhaust gas flow is conducted via the HC adsorber that absorbs and/or chemically adsorbs unburned hydrocarbons and hydrocarbons not converted by the main catalytic converter that is not yet ready for operation. As soon as the main catalytic converter has reached its operating temperature and ensures sufficient HC conversion, the exhaust gas flow is directed into the main passage. As a result of the heating up that takes place, hydrocarbons desorb from the HC adsorber and are returned to engine combustion via the return passage.
A similar concept is known that also uses the concentric arrangement of exterior HC adsorber and interior main duct, however the main catalytic converter is not provided upstream but downstream of the HC adsorber in this design. The return passage described in US 2002/0132726 A1 can be eliminated in this design. Instead, the desorbed hydrocarbons are converted by the downstream main catalytic converter.